The present invention relates to a tip-changeable rotary cutting tool.
FIGS. 9 and 10 show a conventional tip-changeable rotary cutting tool. The tool comprises a columnar holder 1 having at its front-end a groove 2 that cuts across its center, a tip 5 inserted into the groove 2 and supported by jaws 3 at the front end of the holder, and cutting edges 6 provided axis-symmetrically at the edge of the tip 5. The jaws 3 are tightened by clamping screws 7 that penetrate through the holes in the tip 5. This type of tool has been disclosed by the published Japanese patent and utility-model applications Tokukoushou 64-1243, Jitsukaihei 6-53012, Tokukaihei 8-252714, Tokukaihei 8-294814, etc. The tool shown in FIGS. 9 and 10 are a ball-end mill. There are various end mills and drills having a structure similar to that of the ball-end mill.
The present applicant has proposed a cutting tool in the published Japanese patent application Tokuganhei 9-269513. With this cutting tool, each of the two main faces of the tip has a level-changing face that crosses each other in the vicinity of the front end of the tip when viewed in a projected plan view. The level-changing face extends from the vicinity of the center of the front end of the tip toward the side-face of the cutting edge. The level-changing faces are locked by the jaws at the front end of the holder.
The conventional tool shown in FIGS. 9 and 10 supports the rear part of the tip 5 with locking faces (reference faces) 4 of the holder 1. Because the locking faces 4 are far from the cutting edges 6, the tip 5 tends to deviate, pivoting on a point in contact with the locking faces 4 while the tool is cutting. The deviation (runout deviation) readily increases the displacement of the cutting edges 6, reducing the machining precision.
The inside spacing of the groove 2 is restricted in order to secure the stiffness of the jaws 3, so it is difficult to increase the thickness of the tip. This thickness limitation prevents the design of cutting edges forming an optimum shape, such as an S shape when viewed from the front (the S shape is considered to be desirable for a ball-end mill). As a result, most cutting edges have a linear shape with a limitation of two in number. This thickness limitation makes it difficult to improve cutting performance and machining efficiency.
Furthermore, many of the conventional tools have at the innermost portion of the groove the locking faces 4 forming a V shape, an inverted V shape, a circular-arc shape, or a jagged shape. These complex shapes make it extremely difficult to machine the locking faces, hindering the increase of the product precision.
On the other hand, the tool which the present applicant has proposed in Tokuganhei 9-269513 solves the above-described problem because the level-changing faces on the two main faces of the tip are locked by the front ends of the jaws of the holder. This tool, however, locks the tip at the cutting faces for the cutting edges, so it is difficult to provide a large chip pocket. As mentioned above, the level-changing face extends from the vicinity of the center of the front end of the tip toward the side face of the cutting edge. Therefore, the number of cutting edges cannot be increased (an increase in the number increases the machining efficiency). Moreover, each of the two main faces of the tip has a level-changing face. This partial irregularity prevents the tip from being parallel to the supporting plane, so the tip cannot be placed stably on the plane. Consequently, it is difficult to machine the level-changing faces to be locked by the jaws.
The present invention aims to offer a tip-changeable rotary cutting tool that is free from the above-described problems.
In solving the above-described problems, the present invention targets a tip-changeable rotary cutting tool having the following structure:
(a) a holder has at its front end a groove cutting across the center of the holder;
(b) a tip having axis-symmetrically arranged cutting edges is inserted into the groove;
(c) the holder has jaws formed on opposite sides of the groove;
(d) the tip is tightly sandwiched between the jaws; and
(e) the tip can be removed from the jaws for replacement.
The tip-changeable rotary cutting tool of the present invention has the following features:
(a) each of the jaws has a locking face at its front end;
(b) the locking faces face forward obliquely;
(c) each of the two main faces of the tip has an engaging face;
(d) the engaging faces protrude from the tip such that they increase the thickness of the tip;
(e) the engaging faces face backward obliquely; and
(f) the locking faces lock the engaging faces.
In the above description, the term xe2x80x9cforwardxe2x80x9d is used to mean the direction toward the front end of the holder, and the term xe2x80x9cbackwardxe2x80x9d is used to mean the opposite direction.
It is particularly desirable that the holder have the following structure:
(a) the locking face of one jaw and the locking face of the other jaw cross each other at a position a specific distance away from the front end of the tool when viewed in a plan view of the holder;
(b) the specific distance is defined as about one-half the width of the tip;
(c) the two locking faces cross each other at an angle of about 60 to 120 degrees; and
(d) the two locking faces lock the engaging faces each formed on the two main faces of the tip such that they can be engaged with the corresponding locking faces.
In addition, the tip""s front-end portion from the engaging faces to the cutting edges can have a thickness larger than that of the tip""s portion to be inserted into the groove. Furthermore, the tip""s portion to be inserted into the groove can have at least one projected part (machining-stabilizing part) having the same height as that of the tip""s front-end portion. These structures enable the design of the cutting edges to have an optimum shape and the increase of the number of cutting edges.
Because the tool of the present invention locks the tip at the two main faces of the tip, the fulcrum of the runout deviation of the tip (locking portion by the locking face) moves closer to the cutting edges. Consequently, while the cutting is performed, the angular moment exerted on the tip is decreased. This decrease suppresses the runout deviation of the tip. A significantly short distance between the locking portion and the cutting edge decreases the displacements of the cutting edge due to the runout deviation, enabling high-precision machining.
Because the locking faces are provided at the front ends of the jaws, the machining of the locking faces is simplified with a possible increase in the product precision. The position of the locking faces is shifted toward the rear in comparison with that of the tool stated in Tokuganhei 9-269513, so a large chip pocket can be provided.
Because two locking faces slanting in opposite directions are provided axis-symmetrically at the front ends of the jaws, the thickness of the tip""s portion where the cutting edges are formed can be increased without restriction. This increased thickness enables the design of the cutting edges to have a shape superior in cutting performance. Furthermore, it is possible to provide projected portions in the direction to increase the thickness. When cutting edges are provided at the projected portions, the total number of cutting edges can be increased. Because two locking faces are slanted in opposite directions, the tip can secure a centripetal effect when it is set.
In addition, the tip having a machining-stabilizing part can be set stably on the supporting plane. Consequently, the engaging faces of the tip can be machined stably with high precision and at low cost. The machining-stabilizing part can also be used for ensuring the set direction of the tip. As a result, the dimensional precision of the rotary cutting tool can be increased as a whole.